Compact high frequency transportable special antenna system



May 5, 1970 J. H. MULLANEY COMPACT HIGH FREQUENCY TRANSPORTABLE SPIRAL ANTENNA SYSTEM Filed Dec l6. 1966 INSULATED SUPPORT INSULATED SUPPORT- INVENTOR JOHN H. MULLANEY ATTORNY United States Patent US. Cl. 343-745 Claims ABSTRACT OF THE DISCLOSURE This invention relates to an electrically short antenna in relation to its physical dimensions for operation in the frequency range from 2 to 30 megahertz (mHz.). The antenna system contemplated by the subject invention comprises a hybrid type of antenna utilizing a collapsible helical antenna element top fed by the folded-unipole method of feed whereby a feed fold is proximately located outside of, substantially parallel to, and longitudinally of the helical antenna element with the helical antenna element coupled to one end of the feed fold while the other end thereof is capacitively coupled to a. source of radio frequency signal energy. The subject invention acts substantially as a point source of radio frequency and provides an omni directional radiation pattern in both the horizontal and vertical planes.

BACKGROUND OF THE INVENTION There is presently a need for a transportable antenna system which will cover the frequency range of 2 to 30 megahertz (mHz.). This particular need is desirable in many areas of usage but particularly in the military. Such a need has existed since World War II. The antenna required must be physically. small but capable of being erected by not more than two men in five to ten minutes and have technical features that it allows reliable communications between the frequencies of 2 to 30 mHz. The frequency range itself presents a difficult problem inasmuch as the frequency ratio of to 1 is encountered. To use a fixed antenna over this range would require some unusual techniques of tuning or several antennas to properly cover these frequencies. This fact is proven when it is considered that the military actually cover these ranges by using different types of antennas in their point to point services. For close-in communications and for distances of several hundred miles, a foot vertical whip antenna is used as a compromise. This type of antenna, although it is portable, is physically not compatible for use in jungle warfare, because of the nature of the terrain. There is a need then for a small light-weight antenna which can be easily erected in dense foilage. Such an antenna must be easy to tune or load and obtain a reasonable efiiciency over the desired frequency range.

Considering the known prior art apparatus for the purpose aforesaid, generally a series whip antenna is utilized. This type of antenna is in the order of 30 feet in length and is generally mounted on a spring-type of base insulator making it possible to install the antenna on a plate over the ground, on the side of a vehicle or other supporting means. Inasmuch as the frequency range of interest is 2-30 mHz., a 30 foot antenna appears like 22 degrees at 2 mHz. and 330 degrees at 30 mHz. This terminology is adopted by defining a wavelength as being equal to 360 degrees. When it is considered that a 30 foot vertical whip antenna varies from 22 to 330 degrees, it is apparent that at the low frequency range base loading will be required such as the addition of a coil or inductance for resonating, while at the higher frequencies a capacitor in series or tank circuit will be required to reduce the effective or electrical height of the structure. The fact that the antenna of this nature presents difliculty in feeding is only one of its many disadvantages. The most predominant disadvantage and one not generally considered by most of those skilled in the art is the fact that the vertical antenna pattern for an antenna operable between 22 and 330 degrees varies appreciably. As a matter of fact, once the antenna appears 180 degrees /2 wavelength height) multilobes start to appear. The multilobe vertical pattern can either be a substantial advantage or appreciable disadvantage to the user. Because the average user in the field does not plan the frequencies that will be utilized considering the distance and type of antenna employed, the use of the vertical whip antenna presents a serious limitation in communications. Depending upon the frequency chosen, it is possible to create a serious communication problem due to the fact that the vertical pattern for a 30 foot whip antenna at certain frequencies may be so poor that little or no radiation will take place in the desired direction. It must also be kept in mind that between 2 and 30 mHz. there are occasions when ground wave communications will be desired. This requires an antenna generally less than 270 degrees in height to achieve optimum ground wave conditions. It should be noted that maximum radiation at 0 degrees elevation is desired for ground wave communications. Where distances of 100 to 300 miles are involved, the take-ofl? angle or angle of departure varies considerably. At certain frequencies with a taller antenna due to its multilobe effect, it is possible that the antenna might radiate minimum field or even present a null over the pertinent angle which would result in practically no communication or very poor transfer of energy. It should be noted that nothing can be done to improve this situation with a fixed vertical whip antenna. Tuning the whip will not change the vertical antenna pattern so as to alleviate the problem.

It therefore follows for transportable operation an antenna whose vertical antenna pattern approximates a short vertical current element is desirable. Such an antenna moreover should not present multilobe patterns over the entire frequency range but preferably should exhibit a typical vertical pattern for an antenna on the order of degrees or less in vertical height.

Also, there are several patents known to teach the con cepts of helical antennas. For example, the following patents are noted: 1,517,569, I. O. Mauborgne et al.; 1,684,009, H. M. Brown; 2,005,805, H. J. Round; 3,209,358, R. A. Felsenheld. The Mauborgne patent somewhat resembles the embodiment disclosed by the subject invention; however, this reference does not actually suggest the inventive concept hereinafter disclosed. For example, in FIG. 2, the wave coil A is shown mounted in a vertical direction and is connected to the transmitter V through the medium of a lead L, which may be of any suitable length and which may be connected to any point of the wave coil A by means of the connecting point P. Although it appears that this patent discloses a folded unipole antenna, that is not actually the case.

With respect to the Brown patent, this patent merely shows a simple receiving antenna comprising a helix. The same comments may be noted with respect to the Round patent. The Felsenheld patent, on the other hand, discloses a top fed helical antenna; however, it does not disclose the technique of the folded unipole type of antenna as taught by the subject invention.

v detail comprises the use of a collapsible helical antenna fed by means of a folded unipole method of feeding and includes a feed fold running substantially parallel to the helical antenna element outside and longitudinally thereof. The fold is connected at or near the top of the helical antenna element while a source of radio frequency energy is connected to the opposite end. The helical antenna element moreover may be enclosed withn a nonmetallic tube for supporting the antenna or it can be suspended from a suitable supporting structure such as a pole or a tree. The feed fold moreover may embody a simple single conductor which is insulated by some means from the helical antenna element or it may embody a coaxial cable wherein the outer conductor is utilized as the feed conductor while the inner conductor is connected to a source of reference potential such as ground. Furthermore, when desirable, top loading elements may be included with the subject invention for reducing the electrical height of the antenna system even further.

Other advantages of the subject invention will become apparent when the following detailed description is read while considering the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic diagram of one embodiment of the subject invention;

FIG. 2 is a side elevatonal view of a helical antenna element comprising part of the subject invention shown in a compressed or resiliently retracted configuration;

FIG. 3 is an illustrative diagram of a second embodiment of the subject invention;

FIG. 4 is a schematic diagram of the embodiment shown in FIG. 3;

FIG. 5 is a schematic diagram of yet a third embodiment of the subject invention; and

FIG. 6 is a schematic diagram illustrative of means for adjusting the feed point or radiation impedance of the antenna system comprising the subject invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to FIG. 1, there is illustrated a source of radio frequency energy 10 operable in the frequency range of 2 to 30 mHz. Two terminals 12 and .14 are illustrated for coupling electrical energy from the output section of the transmitter 10. Terminal 12 is coupled to a variable capacitor 16 while terminal 14 is connected to a point of reference potential illustrated as ground. The variable capacitor 16 in turn is coupled to an antenna feed terminal 18.

A feed fold conductor 20 comprising a single electrical conductor of predetermined length is located outside and run parallel to the longitudinal dimension of a helical antenna element 22. One end of the feed fold 20 is coupled to the top 24 of the helical antenna element 22 while the opposite end 26 is connected to ground. The helical antenna element 22 moreover comprises a metallic spring-like element which is extended a predetermined length. In the embodiment shown in FIG. 1, the helical antenna element 22 is mounted inside of a tubular element 28 extending the length thereof. Additionally, the feed fold conductor 20 is shown mounted on insulated stand-offs 30 for illustrating the fact that the feed fold runs outside of and substantially the en tire length of the helical antenna element 22. Assuming that the tubular element 28 is comprised of a nonmetallic material, the feed fold could lie on the outside surface of the tubular container. This would provide the necessary insulation between the feed fold 20 and the vertical antenna element 22. If it is desirable to extend the helical antenna element without the use of a tubular container 28 and if the feed fold 20 is comprised of uninsulated wire, then stand-offs 30 must be included to prevent the feed fold 20 from shorting against the helical antenna element 22.

Referring now to FIG. 2, the helical antenna element 22 is shown as being comprised of a helical coil of metallie conductor having a generally rectangular cross-section. The ends 24 and 26 of the helical antenna element 22 comprises a configuration which is referred to as edge wound configuration. The FIG. 2 configuration shows the helical antenna element 22 in a retracted or collapsed condition indicating that it is of a resilient nature and can be extended at will, depending upon the needs of the particular application, from the retracted position of FIG. 2 to the extended position of FIG. 3. It should also be pointed out, however, that although a polygonal cross-section is preferred, a circular cross-sectional material could be utilized when desired.

In the configuration shown in FIG. 3, the transmitter 10 is coupled to an antenna system contemplated by the subject invention wherein the feed fold 20 comprises a coaxial cable having an inner and outer conductor (not shown), a helical antenna element 22 suspended at one end from a support such as a tree by means of a supporting strap 32 and an insulating member 34. The feed fold 20 is separated from the helical element 22 by means of the stand-offs 30 and the feed fold is adapted to run substantially parallel to the helical antenna as described previously. The transmitter 10 feeds electrical energy into the feed fold 20 at the end connected to the antenna by means of terminal 18. The feed fold in turn couples the radio frequency energy to the top 24 of the helical antenna element 22 providing what is referred to as a top fed antenna system and also along its length by means of the capacity between the fold and the helix. This configuration is similar to a folded unipole antenna; however, the radiating element is comprised of the helical element 22. Furthermore, FIG. 3 is also adapted to include a top loading element comprising the top loading conductors 36 which are commonly coupled to the insulator member 34 at the common terminal 35 while their respective distal ends extend outwardly therefrom and are attached to supports by means of the insulators 38 and attachment members 40.

This configuration is shown in schematic representation in FIG. 4. Referring now to FIG. 4, the transmitter 10 includes a variable capacitor 16 internally for purposes of adjusting the feed point impedance. The other output terminal 14 is coupled to ground. The input terminals 42 and 44 are adapted to receive an input signal thereacross. The feed fold 20 comprises an inner conductor 46 and an outer conductor 48. The outer conductor 48 is connected at one end to the output terminal 18 while the opposite end is connected to the upper terminal 24 of the helical antenna element 22. The inner conductor 46 of the feed fold 20 is connected at one end to ground which is common to the output terminal 14 while the opposite end thereof is connected to the common connection 35 of the top loading wires 36. The opposite end of the helical antenna element 22 is also coupled to ground.

The configuration in FIG. 4, then, discloses an antenna system which includes a helical antenna element, a feed fold 20 parallel thereto, and top loading elements 36 coupled together such that the feed fold element runs outside of the helical element 22 with the outer conductor 48 of the feed fold connected to the source of radio frequency energy 10 and the top 24 of the helical antenna element. The top loading elements are coupled to the helical antenna element 22 by means of the capacity of the coaxial cable comprising the feed fold element 20, i.e., the inner conductor 46 is capacitively coupled to the outer conductor 48 which in turn is directly coupled to the upper terminal 24 of the helical antenna element 22.

The subject invention electrically provides an impedance which includes a resistance value (R) ranging from 20 to 200 ohms and an inductive reactance (+jX). The variable capacity 16 is used to offset the inductive reactance to make the system appear purely resistive. Furthermore, the system acts as if it were a quarter-wave antenna while physically its length is substantially less than a quarter-wave length. For example, a helical antenna element for operation over a frequency range of 2 to 30 mHz. is of the order of 9 feet in length. Also, the present invention does not provide the usual voltagecurrent distribution of a vertical antenna but acts substantially like a point source providing an omni-directional radiation pattern along both the horizontal and vertical planes. Although the top loading element comprising the top loading wires 36 are not essential to the operation of the subject invention, the top loading increases the effective height of the structure so that its equivalent height is quite similar to a 30 foot whip antenna over the entire frequency range. The coaxial cable utilized as the feed fold 20 allows the high inductive reactance of the helical antenna element 22 to be materially reduced so as to provide a better coefficient of coupling.

FIG. discloses another embodiment for providing an alternate method of coupling radio frequency energy to the antenna system. The configuration shown connects the upper terminal 24 of the helical antenna element 22 directly to the common connection 35 of the top loading elements 36. Moreover, the center conductor 46 and the outer conductor 48 of the fold conductor 20 are also connected together at the upper terminal 24 of the helical antenna element 22. The opposite end of the center conductor 46 is coupled to ground which is common to the opposite end of the helical antenna element 22 and the outer conductor 48 of the feed fold elements 20 is coupled to the terminal 18 which is the hot lead of the transmission line feeding the radio frequency energy to the antenna system. The variable capacitance 16 is also included as shown heretofore to provide a means of cancelling the +jX reactancefor providing a purely resistive impedance. Capacity coupling of the RF energy takes place over the entire length of the helical antenna element 22 between the outer conductor 48 and the helix even though the upper extremities of the conductors are shorted together, that is, the top loading elements 36 and the inner and outer conductors 46 and 48, respectively.

When desirable, the impedance of the helical antenna element 22 can be varied by means of the insertion of either a ferrite or metallic rod into one end of the helical element as shown in FIG. 6. FIG. 6 discloses a helical antenna element 22 located within a tubular container 28 being fed at the upper end 24 by means of a feed fold 20 comprising an insulated wire which runs substantially parallel to the helical antenna element 22 outside thereof and over its length. At the lower end of the helical antenna element 22, an opening is provided in the tubular container 28 such that a rod 50 can be moved in and out of the helical antenna element 22 a desired distance. Depending upon the material used, the impedance can be increased or decreased. For example, if a ferrite rod is utilized, the impedance will be decreased, whereas if a metallic rod is inserted, the impedance will be increased. It has also been determined that if a shorting bar 52 is coupled between the fold wire 20 which has had its insulation removed and one or more turns of the antenna element 22, the impedance can also be varied. It is possible, then, to tune the feed point impedance of the entire system by use of a shorting stub 52.

What has been illustrated therefore is an antenna system which is transportable, simple in construction and one which can be erected in a matter of minutes by a single individual. It should also be noted that this antenna system can be erected in an area where there is dense foliage, where a 30 foot whip antenna would be practically unusable. This obtains a great advantage because the subject invention, over the range desired, is approximately 9 feet in length; It should also be pointed out that where use is desired in an area where there are no tree or foliage to which the helical antenna element and/or the top loading wires can be attached, a noninsulated pole can be extended through the center of the antenna element or parallel thereto for physical support. The top loading wires would then be tied to the top in the usual manner and extend down to a ground stake with an appropriate insulator at the end thereof.

The present invention moreover is an improvement over prior art apparatus inasmuch as for desired frequency range, the so-called multilobe effect is not present. This is due to the fact that a resonance condition over the entire frequency range of 230 mHz. is achieved.

While there has been shown and described what is at present considered to be the preferred embodiment of the invention modifications thereto will readily occur to those skilled in the art. It is not desired therefore that the invention be limited to those specific arrangements shown and described, but it is to be understood that all equivalents, alterations, and modifications within the spirit and scope of the invention are herein meant to be included.

What is claimed is:

1. A top fed series antenna system comprising in combination: a helical antenna element having a physical length less than a quarter wave length for the desired frequency of operation and having one end and an opposite end; means for coupling one end of said helical antenna element to a point of reference potential; a feed fold having one end coupled to the opposite end of said helical antenna element, being proximately located outside of and running uninterrupted parallel to said heilcal antenna element and longitudinally thereof; means for electrically insulating said feed fold from said helical antenna element intermediate the ends thereof; said feed fold having an opposite end adjacent said one end of said helical antenna element; circuit means for supplying a radio frequency signal; and a capacitive reactance series connecting said circuit means to the opposite end of said feed fold to supply a radio frequency signal to and to resonate the series connected feed fold and helical antenna element.

2. A top fed antenna system as set forth in claim 1 in which said means for electrically insulating said feed fold from said helical antenna element comprises an electrically insulated tube member enclosing said helical antenna element.

3. A top fed antenna system comprising in combination: a helical antenna element having a physical length less than a quarter wave length for the desired frequency of operation; means for coupling one end of said helical antenna element to a point of reference potential; a feed fold having one end coupled to the opposite end of said helical antenna element, being proximately located outside of and running parallel to said helical antenna element and longitudinally thereof; means for electrically insulating said feed fold from said helical antenna element intermediate the ends thereof; circuit means coupled to the opposite end of said feed fold for supplying a radio frequency signal thereto; and a shorting bar placed between said feed fold and said helical antenna element at a determined location intermediate the ends thereof for adjusting the feed point impedance of said antenna system.

4. The apparatus as defined in claim 3, and including a rod inserted within said helical antenna element a predetermined length for changing the feed point impedance of said antenna system.

5. Apparatus as defined by claim 3 and additionally including a rod of ferro-magnetic or metallic material inserted within said helical antenna element a predetermined distance for changing the radiation impedance of said antenna system.

6. A top fed antenna system comprising in combination: a helical antenna element having a physical length less than a quarter wave length for the desired frequency of operation and having one end and an opposite end; means for coupling said one end of said helical antenna element to a point of reference potential; a coaxial transmission line feed fold having a first and a second conductor and including means for connecting said first conductor at one end to said helical antenna element; said coaxial transmission line feed fold being proximately located outside of and running parallel to said helical antenna element and longitudinally thereof; means for electrically insulating said coaxial transmission line feed fold from said helical antenna element intermediate the ends thereof; said first conductor having an opposite end;

circuit means coupled to the opposite end of said first conductor for supplying a radio frequency signal thereto; and second means for coupling said second conductor to said point of reference potential.

7. The apparatus as defined by claim 6 including a capacitive reactance coupled in series with said circuit means for applying a radio frequency signal to said first conductor and helical antenna element.

8. A top fed antenna system comprising in combination: a helical antenna element having a physical length less than a quarter wave length for the desired frequencyof operation and having one end and an opposite end; means for coupling said one end of said helical antenna element to a point of reference potential; a coaxial transmission line feed fold having a first and a second conductor; means for coupling said first conductor at one end to said opposite end of said helical antenna element; said coaxial transmission line feed fold being proximately located outside of and running parallel to said helical antenna element and longitudinally thereof; means for electrically insulating said coaxial transmission line feed fold from said helical antenna element intermediate the ends thereof; a plurality of top loading elements located above said helical antenna element and extending outwardly therefrom; said first conductor having an opposite end; circuit means coupled to the opposite end of said first conductor for applying a radio frequency signal thereto; and second circuit means for coupling one end of said second conductor of said coaxial transmission line feed fold to said top loading elements and the other end thereof to said point of reference potential.

9. A top fed antenna system comprising in combination: a helical antenna element having a physical length less than a quarter wave length for the desired frequency of operation and having one end and an opposite end; means for coupling said one end of said helical antenna element to a point of reference potential; a feed fold comprised of a coaxial cable having a first conductor and a. second conductor; said feed fold being proximately located outside of and running parallel to said helical antenna element and longitudinally thereof; means for electrically insulating said coaxial cable from said helical antenna element intermediate the ends thereof; a plurality of top loading elements commonly connected at one end to said opposite end of said helical antenna element and extending outwardly from said helical antenna element; circuit means for supplying a radio frequency signal; another means connecting said circuit means to said first conductor at one end thereof; second means for connecting the other end of said first conductor to said opposite end of said helical antenna element; and third means for coupling the second conductor of said coaxial cable to the point of reference potential.

10. A top fed collapsible antenna system comprising in combination: a helical antenna element constructed of spring material so that it is extendible and retractable, said helical antenna element when extended having a physical length less than a quarter wave length for the desired frequency of operation and having one end and an opposite end, means coupling said one end of said helical antenna element to a point of reference potential, a feed fold having one end coupled to said opposite end of said helical antenna element, said feed fold being proximately located outside of and running uninterrupted parallel to said helical antenna element and longitudinally thereof; means electrically insulating said feed fold from said helical antenna element intermediate the ends thereof, circuit means coupled to the opposite end of said feed fold for supplying a radio frequency signal thereto, and said circuit means including a capacitive reactance connected in series circuit with said feed fold and helical antenna element.

References Cited UNITED STATES PATENTS 2,931,034 3/1960 Harrison et al. 343-750 3,103,011 9/1963 Sefiey 343-845 3,179,941 4/1965 Harris et al. 343-895 3,209,358 9/ 1965 Felsenheld 343-895 3,295,137 12/1966 Fenwick et al 343830 3,403,405 9/1968 Barrar et al. 343845 ELI LUBERMAN, Primary Examiner US. Cl. X.R. 343-895 

